In a magnetic recording device in which a read head comprises a magnetoresistive (MR) sensor, there is a constant drive to increase recording density. However, as areal density becomes higher, there is greater difficulty in designing reader sensors with improved signal to noise ratio (SNR) to facilitate reading data from the disk. Recently, magnetic recording heads having two readers that are stacked one above the other in the down-track direction have been introduced and implemented in products in order to enhance ADC. Existing designs utilize the two readers as independent readers. In today's design, the two readers are made sequentially and will have the typical alignment errors that are fundamentally limited by process capabilities. Also, with each reader having a tunnel barrier between a free layer and a pinned layer, and a top shield and bottom shield, there is a reader to reader separation (RRS) in the down-track direction. This RRS, and misalignment in the cross-track direction limits the region on the disk where the readers are aligned well enough to allow joined signal processing using the read back signal from the two sensors read at the same time on the same data track. This limits the total integrated capacity gain that is possible with a dual reader.
A longitudinal biasing scheme is typically used in a read head design to keep the free layer in a stable orientation in the absence of the external magnetic field. Bias films of high coercivity or soft bias also known as junction shields, are abutted against the edges of the MR sensor and particularly against the sides of the free layer. As the critical dimensions for MR sensor elements become smaller with higher recording density requirements, the free layer becomes more volatile and more difficult to bias. Top and bottom magnetic shields with in-plane magnetization are often used to ensure the MR sensor will only respond to a local magnetic field. Free layer magnetization is sensitive to domain wall motion in the bottom and top shields, which may lead to increased noise, reducing the SNR of the reader sensor and cause failure in decoding data from the media.
Thus, a new dual read head structure is needed wherein RRS is substantially reduced and cross-track alignment between reader sensors is improved to enable a maximum gain in ADC while maintaining acceptable SNR.